The present invention relates to laser light sources, and, more particularly, to frequency conversion in optical fibers using a laser as an optical pump source.
Systems such as image projection systems, for example, would benefit from compact, high luminous efficacy, long life, and narrow divergence angle light sources. Present short gap arc sources (gap length 1 mm to 1.8 mm) consume 100 to 270 watts of electrical power, with luminous efficacy of 2 to 5 lumens/watt in projection systems. The life time of these devices is on the order of 50 to 10,000 hours.
Semiconductor lasers are relatively small, with emission areas on the order of 5 xcexcmxc3x971 xcexcm, and beam divergence angles of approximately 12xc2x0 for the 5 xcexcm dimension, and 40xc2x0 for the 1 xcexcm dimension. These semiconductor devices may have optical conversion efficiencies in excess of 50%. The high conversion efficiency gives these devices a luminous efficacy that is 5 to 10 times that of the arc sources in projection systems. Further, low beam divergence angles allow for higher collection efficiency by condenser lenses. As a result, a total of only a few watts from such a laser source may be sufficient for a projection system. A single single-mode semiconductor light source typically provides less than one watt of output power. Several of these devices, however, may be fabricated in a single step as a closely spaced monolithic array to provide the power for a projection system.
Presently direct band gap (low-cost) semiconductor lasers, with sufficient power, are only available in the wavelengths corresponding to red and infra-red, using a III-V compound material (typically based on gallium arsenide or indium phosphide). Direct band gap lasers for wavelengths corresponding to blue and green may require the use of II-VI gallium nitride-based material, but these devices are currently unable to provide sufficient power at room temperature. Several methods to make blue and green lasers are being studied. These typically involve up-conversion of semiconductor infra-red lasers.
There are currently two methods for generating blue and green light by frequency up-conversion. The first method uses a rare-earth-doped crystal to double the frequency of the laser light beam. Power levels as high as 3 watts have been obtained by this method in the green wavelengths (≈532 nm), and as high as 0.5 watt in the blue wavelengths (≈457 nm). It may be difficult, and therefore costly, to mass produce laser light sources according to this method. Moreover, the light sources produced by this method may be inefficient and may not operate well at room temperature.
The second method is a two-photon process in certain phosphors. Red, green, and blue light have been generated using a two-photon process by pumping appropriately doped phosphors using light at 980 nm. This method may have relatively low conversion efficiency, and may not produce relatively high visible power in small areas. Furthermore, the resulting emission is Lambertian. Therefore, collection efficiency to a small light valve would be relatively small. This type of approach may be appropriate if the phosphor is embedded in the projection screen. In a configuration of this type, however, it may be difficult to maintain alignment between the projector and the screen, without the use of sensors on the screen and a feedback system.
The present invention is embodied in an optical system for generating light. A doped optical fiber is used for converting energy of light having a predetermined wavelength, supplied by an optical pump, to light having another wavelength. A first reflector is positioned at a first facet of the doped optical fiber, and a second reflector is positioned at a second facet of the doped optical fiber. The first and second reflectors selectively reflect light having the other wavelength.
According to one aspect of the invention, the optical system further comprises another optical fiber, and a third reflector. The second reflector couples a first facet of the other optical fiber to the second facet of the doped optical fiber. The first reflector substantially reflects light having the other wavelength and substantially transmits light having the predetermined wavelength. The second reflector partially reflects light having the other wavelength and substantially transmits light having the predetermined wavelength. The other optical fiber provides an optical path for light having the predetermined wavelength. The third reflector is positioned at the second facet of the other optical fiber. The third reflector substantially reflects light having the predetermined wavelength and substantially transmits light having the other wavelength.
According to yet another aspect of the invention, the optical system includes: an optical gain medium, first through fourth fibers, a first reflector, and a second reflector. The optical gain medium, has a first facet and a second facet, and provides light at a first frequency via its first facet to a first facet of the first fiber. The second fiber provides an optical path and converts energy of light at the first frequency to light at a second frequency. The first reflector is positioned at the first facet of the second fiber to couple a second facet of the first fiber to a first facet of the second fiber. The first reflector substantially reflects light at the second frequency. The third fiber provides an optical path for light at the first frequency and is coupled to the second facet of the optical gain medium. The second reflector is positioned at the second facet of the second fiber. The second reflector couples the first facet of the third fiber to the second facet of the second fiber and substantially reflects light at the second frequency. The fourth fiber is fused to the second fiber and provides an output path for light at the second frequency.
According to another aspect of the invention, the optical system further comprises another optical pump coupled to the doped optical fiber. The other pump also generates light a the predetermined wavelength. The optical pump is coupled to the optical fiber, adjacent to the first facet of the doped optical fiber, by a WDM (wavelength division multiplexer). The other optical pump is coupled to the optical fiber, adjacent to the second facet of the doped optical fiber, by another WDM.